CN115408875A - Modelica language-based dynamic simulation method for lithium bromide absorption refrigeration system - Google Patents
Modelica language-based dynamic simulation method for lithium bromide absorption refrigeration system Download PDFInfo
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- AMXOYNBUYSYVKV-UHFFFAOYSA-M lithium bromide Chemical compound [Li+].[Br-] AMXOYNBUYSYVKV-UHFFFAOYSA-M 0.000 title claims abstract description 98
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- 238000005057 refrigeration Methods 0.000 title claims abstract description 50
- 238000010521 absorption reaction Methods 0.000 title claims abstract description 49
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- 238000002474 experimental method Methods 0.000 claims abstract description 5
- 239000006096 absorbing agent Substances 0.000 claims description 87
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 70
- 239000003507 refrigerant Substances 0.000 claims description 41
- 238000004134 energy conservation Methods 0.000 claims description 31
- 239000000498 cooling water Substances 0.000 claims description 26
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Abstract
The invention relates to a dynamic simulation method of a lithium bromide absorption refrigeration system based on Modelica language, which comprises the following steps: modeling each component of the lithium bromide absorption refrigeration system through a Modelica modeling language to form a component model; respectively establishing connectors aiming at different physical domains in the system; adding corresponding connectors on each component model, and correspondingly connecting the connectors according to system characteristics to form a dynamic simulation model so as to realize variable transmission among the component models; setting system structural parameters according to input parameters under the design working condition, carrying out simulation calculation under the design working condition, and correcting the dynamic simulation model by adjusting the structural parameters; and performing a step change experiment on the input parameters of the dynamic simulation model to obtain the dynamic response of the output parameters of the lithium bromide absorption refrigeration system. The invention reduces the complexity of the model, improves the calculation precision and the running speed, and provides a basis for the design of the lithium bromide absorption refrigeration system and the controller.
Description
Technical Field
The invention relates to the technical field of numerical simulation, in particular to a dynamic simulation method of a lithium bromide absorption refrigeration system based on Modelica language.
Background
The lithium bromide absorption refrigeration system can effectively utilize low-grade waste heat, is environment-friendly, and is widely applied to the fields of chemical industry, pharmacy, steel and the like. A lithium bromide absorption refrigeration cycle generally consists of a refrigerant circuit, a solution circuit, a heat source circuit, a cooling water circuit, and a chilled water circuit. A heat source loop consisting of a generator, a heat source and the like drives the unit to operate; a cooling water loop formed by equipment such as an absorber, a condenser, a cooling water pump and the like is used for removing solution absorption heat and steam condensation heat; the refrigeration water circulation formed by the evaporator, the refrigeration water pump and other equipment generates refrigeration capacity. The refrigerant steam is generated by the generator, cooled and condensed by the condenser, enters the throttle valve for pressure reduction and throttling, then enters the evaporator for evaporation and heat absorption, and finally is absorbed by the dilute solution in the absorber. And the dilute solution in the absorber absorbs water and then enters the generator through the solution pump and the solution heat exchanger, and the generated concentrated solution returns to the absorber through the solution heat exchanger and the solution valve to form a solution loop. At present, the technical problems of preventing the lithium bromide absorption refrigeration system from operating efficiently and safely comprise: because of the large thermal inertia and long conditioning time of the absorption refrigeration system itself, the cost of conducting experimental research on it is large. In addition, changes in the external environment and changes in refrigeration requirements during system operation cause the unit to frequently operate in off-design conditions. Therefore, in order to know the operation characteristics of the lithium bromide absorption refrigerating unit in a dynamic process, an efficient and economic controller needs to be designed, and a numerical simulation model which is high in precision and easy to expand and transplant needs to be designed.
The modeling method of the conventional numerical simulation usually uses a programming language based on command lines, such as C, fortran. The modeling method generally needs to determine a separate solving algorithm according to different system structures, and brings inconvenience when the simulation system is debugged and modified. Moreover, most of the traditional modeling languages are causal modeling, that is, different simplification methods may be needed to solve for different component models in the simulation system. The characteristics improve the modeling complexity of the dynamic simulation model and reduce the reusability of the model.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a dynamic simulation method of a lithium bromide absorption refrigeration system based on Modelica language, aiming at reducing the modeling complexity and improving the precision of simulation calculation.
The technical scheme adopted by the invention is as follows:
a dynamic simulation method of a lithium bromide absorption refrigeration system based on Modelica language comprises the following steps:
the method comprises the steps that a lithium bromide absorption refrigeration system is split into different components according to functions, modeling is carried out on each component through a Modelica modeling language based on heat exchange, mass conservation, energy conservation and momentum conservation to form a component model, wherein the component model comprises a generator model, an absorber model, a phase-change heat exchanger model, a solution pump and a valve model, and the phase-change heat exchanger model is suitable for describing an evaporator and a condenser;
respectively establishing connectors aiming at different physical domains in the lithium bromide absorption refrigeration system, and calculating variables of the different physical domains and transmitting the variables among the component models;
adding corresponding connectors on each component model, and correspondingly connecting the connectors of the same type in each component model according to the structure of the lithium bromide absorption refrigeration system to form a dynamic simulation model;
setting structural parameters of the system according to input parameters under the design working condition, carrying out simulation calculation under the design working condition, comparing a simulation result with an output result of an actual system, verifying a dynamic simulation model, and correcting the dynamic simulation model by adjusting the structural parameters;
carrying out dynamic simulation research: and performing a step change experiment on the input parameters of the dynamic simulation model to obtain the dynamic response of the output parameters of the lithium bromide absorption refrigeration system.
The further technical scheme is as follows:
the modeling of each component through the Modelica modeling language comprises the following steps:
establishing a generator model:
establishing a generator shell side dynamic model according to an energy conservation equation of the generator shell side and a heat exchange equation of the shell side and a pipe wall, wherein a driving work term is considered in the energy conservation equation of the generator shell side:
wherein M is gen ，h gen ，p gen Respectively the accumulated mass, solution specific enthalpy and pressure in the generator;
respectively, dilute solution mass flow, concentrated solution mass flow and generated steam mass flow; v gen Is the generator volume; h is a total of weak ，h strong ，h v,gen Respectively dilute solution specific enthalpy, concentrated solution specific enthalpy and generated steam specific enthalpy;
the heat exchange quantity between the shell side of the generator and the pipe wall is obtained; t represents time;
establishing a generator tube side dynamic model according to an energy conservation equation of a generator tube side fluid and a heat exchange equation of the tube side fluid and a tube wall;
establishing a generator pipe wall dynamic model according to an energy conservation equation of the generator pipe wall;
and combining the generator shell side dynamic model, the generator tube side dynamic model and the generator tube wall dynamic model into the generator model.
The modeling of each component through the Modelica modeling language comprises the following steps:
establishing an absorber model:
establishing an absorber shell side dynamic model according to an energy conservation equation of the absorber shell side and a heat exchange equation of a shell side fluid and a pipe wall, wherein a pushing work term is considered in the absorber shell side energy conservation equation:
wherein M is abs ，h abs ，p abs Respectively the accumulated mass, the solution specific enthalpy and the pressure in the absorber;
respectively, dilute solution mass flow, concentrated solution mass flow and absorbed steam mass flow; v abs Is the absorber volume; h is v,abs To absorb specific enthalpy of steam;
the heat exchange quantity between the shell side of the absorber and the tube wall is obtained; t represents time;
establishing an absorber tube side dynamic model according to an energy conservation equation of absorber tube side fluid and a heat exchange equation of the tube side fluid and a tube wall;
establishing an absorber tube wall dynamic model according to an energy conservation equation of the absorber tube wall;
and combining the absorber shell side dynamic model, the absorber tube side dynamic model and the absorber tube wall dynamic model into the absorber model.
The modeling of each component through the Modelica modeling language comprises the following steps:
establishing a phase change heat exchanger model:
dividing the phase change heat exchanger into a plurality of control volumes with equal length along the fluid flow direction, wherein each control volume follows mass conservation, energy conservation and momentum conservation, and the control equation of the control volume at the refrigerant side of the phase change heat exchanger is as follows:
wherein, V r,i A control volume on the refrigerant side; ρ is a unit of a gradient r,i ，p r,i ，h r,i Density, pressure and specific enthalpy in the ith control volume, respectively, of the refrigerant side;
and
mass flow rates into and out of the ith refrigerant side control volume, respectively;
and
specific enthalpy of refrigerant inflow and outflow, respectively;
and
the refrigerant pressure of the inflow and outflow, respectively; q r,i The heat exchange quantity of the refrigerant side and the shell; t represents time.
The modeling of each component through the Modelica modeling language comprises the following steps:
establishing a solution heat exchanger model:
dividing the solution heat exchanger into a plurality of control volumes of equal length along the fluid flow direction, each control volume following conservation of mass, energy and momentum, the governing equation for the control volume at the refrigerant side of the solution heat exchanger being:
wherein,
and
mass flow of solution into and out of the ith control volume, respectively;
and
mass fractions of the influent and effluent solutions, respectively; v sol,i Is the control volume of the solution heat exchanger;
and
specific enthalpy of solution inflow and outflow, respectively; rho sol,i ，p sol,i ，h sol,i Density, pressure and specific enthalpy in the ith control volume in the solution heat exchanger, respectively;
and
the pressure of the solution flowing in and out respectively; q sol,i The heat exchange quantity of the ith control volume in the solution heat exchange; t represents time.
The modeling of each component through the Modelica modeling language comprises the following steps:
and establishing a solution pump and valve model through a flow calculation formula of the solution pump and the valves, wherein the valves comprise a solution regulating valve and a refrigerating machine throttling valve.
The connector comprises a solution connector, a water connector and a heat connector, wherein a circulation variable in the solution connector is solution mass flow, a potential variable is pressure, a flow variable is temperature and mass fraction of the solution, a circulation variable in the water connector is mass flow of refrigerant, a potential variable is pressure, a flow variable is specific enthalpy of the refrigerant, a circulation variable in the heat connector is heat flow, and a potential variable is temperature.
The input parameters comprise the water flow of a generator tube side heat source, the water inlet temperature of the generator tube side heat source, the chilled water inlet flow and the chilled water inlet temperature of an evaporator heat exchange medium side, the cooling water flow of an absorber tube side, the cooling water inlet temperature of the absorber tube side, the opening degree of a valve and the rotating speed of a solution pump; the output parameters comprise the heat exchange quantity in the condenser, the heat exchange quantity in the evaporator, the heat exchange quantity in the solution heat exchanger, the chilled water outlet temperature at the heat exchange medium side of the evaporator, the heat exchange quantity between the shell side of the generator and the tube wall, and the heat exchange quantity between the shell side of the absorber and the tube wall.
The invention has the following beneficial effects:
the invention reduces the complexity of the model and improves the simulation calculation precision and the running speed.
According to the invention, the result of the dynamic simulation model can be matched with the output value of the actual system by adjusting the model structure parameters under the design working condition, so that the accuracy of the dynamic simulation model is ensured.
The invention can obtain the dynamic response curve of the output parameters of the dynamic simulation model by using a step response experiment, is convenient for carrying out the analysis of the dynamic characteristics of the system and provides reference for the design of the system and the controller.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
Drawings
Fig. 1 is a schematic structural diagram of a lithium bromide absorption refrigeration system according to an embodiment of the present invention.
Fig. 2 is a graphical interface of a dynamic simulation model of a lithium bromide absorption refrigeration system constructed based on a modeica modeling language according to an embodiment of the present invention.
Fig. 3 is a comparison diagram of chilled water outlet temperature, system energy efficiency and actual system output obtained by simulation of a dynamic simulation model of a lithium bromide absorption refrigeration system in the embodiment of the present invention.
Fig. 4 is a dynamic response diagram of the steam flow and the generator power generated by the generator when the heat source water flow and the temperature change in a step manner in the dynamic simulation research process according to the embodiment of the invention.
FIG. 5 is a dynamic response diagram of chilled water outlet temperature and system energy efficiency when the heat source water flow and temperature have step changes in the dynamic simulation research process according to the embodiment of the present invention.
FIG. 6 is a diagram of the dynamic response of the generator steam flow and absorber power when the cooling water flow and temperature have step changes during the dynamic simulation study of the embodiment of the present invention.
FIG. 7 is a dynamic response diagram of chilled water outlet temperature and system energy efficiency when the cooling water flow and temperature have step changes in the dynamic simulation research process according to the embodiment of the present invention.
Detailed Description
The following describes embodiments of the present invention with reference to the drawings.
The dynamic simulation method for the lithium bromide absorption refrigeration system based on the Modelica language in the embodiment comprises the following steps:
s1, splitting a lithium bromide absorption refrigeration system into different components according to functions, and modeling each component through a Modelica modeling language based on heat exchange, mass conservation, energy conservation and momentum conservation to form a component model, wherein the component model comprises a generator model, an absorber model, a phase-change heat exchanger model, a solution pump and a valve model, and the phase-change heat exchanger model is suitable for describing an evaporator and a condenser.
The structure of the lithium bromide absorption refrigeration system of the present embodiment is shown in fig. 1. The lithium bromide absorption refrigeration cycle is composed of a refrigerant loop, a solution loop, a heat source loop, a cooling water loop and a chilled water loop. A heat source loop consisting of a generator and heat source water drives the unit to operate; a cooling water loop formed by an absorber, a condenser, a cooling water pump and the like is used for removing solution absorption heat and steam condensation heat; the refrigeration water circulation formed by the evaporator, the refrigeration water pump and other equipment generates refrigeration capacity. Refrigerant steam is generated by a generator, is cooled and condensed by a condenser, enters a throttle valve for pressure reduction and throttling, then enters an evaporator for evaporation and heat absorption, and is finally absorbed by dilute solution in an absorber. The dilute solution in the absorber absorbs water and then enters the generator through the solution pump and the solution heat exchanger, and the generated concentrated solution returns to the absorber through the solution heat exchanger and the solution valve to form a solution loop.
Wherein the solution is lithium bromide and the refrigerant is water. Setting the power of a generator to be 9kW, the power of an absorber to be 8.6kW, the power of a condenser to be 7.4kW and the power of an evaporator to be 7kW; the designed input temperature of the heat source water is 90 ℃, and the flow rate is 0.82m 3 H; the design input temperature of the cooling water is 30 ℃, and the flow rate is 1.34m 3 H; the design input temperature of the chilled water is 18 ℃, and the flow rate is 2.2m 3 /h。
Specifically, the modeling of each component through a Modelica modeling language to form a component model includes:
s11, establishing a generator model:
according to an energy conservation equation of the shell side of the generator and a heat exchange equation of the shell side and the pipe wall, a dynamic model of the shell side of the generator is established as follows:
in the formula (1), M gen ，w gen ，h gen ，p gen Respectively, the accumulated mass (of the lithium bromide solution), the lithium bromide mass fraction, the solution specific enthalpy and the pressure in the generator;
respectively, dilute solution mass flow, concentrated solution mass flow and generated steam mass flow; w is a weak ，w strong Respectively representing the mass fraction of the dilute solution and the mass fraction of the concentrated solution; v gen Is the generator volume; h is weak ，h strong ，h v,gen The specific enthalpy of the dilute solution, the specific enthalpy of the concentrated solution and the specific enthalpy of generated steam; p is a radical of strong The pressure of the concentrated solution;
the heat exchange quantity between the shell side of the generator and the pipe wall is obtained;
and
the heat transfer coefficient and the heat transfer area of the shell side of the generator are respectively;
and
the generator tube wall temperature and the shell side temperature, respectively; t represents time.
In equation (1), the push work term is considered in the generator shell side energy conservation equation:
by promoting the setting of the work term, the influence of the pressure change in the generator on the energy equation can be better, and the dynamic state of the generator can be more accurately described.
According to an energy conservation equation of the generator tube side fluid and a heat exchange equation of the tube side fluid and the tube wall, a generator tube side dynamic model is established as follows:
in the formula (2), the reaction mixture is,
the heat exchange quantity between the tube side of the generator and the tube wall is obtained;
and
the heat transfer coefficient and the heat transfer area of the generator tube side are respectively;
and
respectively the outlet temperature and the inlet temperature of the water of the heat source at the tube side of the generator;
is the heat source water flow at the tube side;
the specific heat capacity of the heat source water at the tube side;
the tube side heat source water density;
the volume was controlled for the tube side.
According to the energy conservation equation of the generator pipe wall, a generator pipe wall dynamic model is established as follows:
in the formula (3), the reaction mixture is,
is the specific heat capacity of the tube wall of the generator;
is the generator tube wall density;
controlling the volume for the generator tube wall;
is the generator tube wall temperature.
Combining equations (1) through (3) into the generator model.
S12, establishing an absorber model:
according to an energy conservation equation of the shell side of the absorber and a heat exchange equation of fluid and a pipe wall of the shell side, establishing an absorber shell side dynamic model as follows:
in the formula (4), M abs ，w abs ，h abs ，p abs Respectively the accumulated mass (of the lithium bromide solution), the mass fraction of the lithium bromide, the specific enthalpy of the solution and the pressure in the absorber;
to absorb the steam mass flow; v abs Is the absorber volume; h is v,abs To absorb specific enthalpy of steam; p is a radical of weak Pressure of dilute solution;
the heat exchange quantity between the shell side of the absorber and the tube wall is obtained;
and
the heat transfer coefficient and the heat transfer area of the absorber shell side are respectively;
and
absorber tube wall temperature and shell side temperature, respectively; t represents time;
in equation (4), the push work term is considered in the absorber shell side energy conservation equation:
by the arrangement of the push work item, the influence of pressure change in the absorber on an energy equation can be better brought, and the dynamic state of the absorber can be more accurately described.
According to an energy conservation equation of the fluid on the tube side of the absorber and a heat exchange equation of the fluid on the tube side and the tube wall, establishing an absorber tube side dynamic model as follows:
in the formula (5), the reaction mixture is,
the heat exchange quantity between the absorber tube side and the tube wall;
and
heat transfer coefficients and heat transfer areas on the absorber tube side, respectively;
and
the outlet temperature and the inlet temperature of the cooling water on the tube side are respectively;
cooling water flow for the tube side;
the specific heat capacity of cooling water at the tube side;
the tube side cooling water density;
the volume was controlled for the tube side.
According to the energy conservation equation of the absorber tube wall, establishing an absorber tube wall dynamic model as follows:
in the formula (6), the reaction mixture is,
is the specific heat capacity of the absorber tube wall;
is absorber tube wall density;
controlling the volume of the absorber tube wall;
is the absorber tube wall temperature.
Combining equations (4) through (6) into the generator model.
Compared with the traditional modeling mode, the modeling mode of the embodiment can modify and debug the generator model/absorber model more conveniently and more conveniently, and mathematical processing on a dynamic equation is not needed.
S13, dividing the phase change heat exchanger into a plurality of control volumes with equal length along the fluid flow direction, wherein each control volume follows mass conservation, energy conservation and momentum conservation, and establishing a phase change heat exchanger model as follows:
in the formula (7), V r,i ,V w,i ,V shell,i The ith control volumes of the refrigerant side, the heat exchange medium side and the shell respectively; rho r,i ，p r,i ，h r,i Density, pressure and specific enthalpy in the ith control volume, respectively, of the refrigerant side;
and
mass flow rates into and out of the ith refrigerant side control volume, respectively;
and
mass flow rates for the inflow and outflow ith media side control volumes, respectively;
and
specific enthalpy of refrigerant inflow and outflow, respectively;
and
the refrigerant pressure of the inflow and outflow, respectively; rho w,i ，T r,i Density and temperature in the ith control volume on the media side, respectively; c p,w Constant pressure heat capacity on the medium side;
and
the temperature of the medium flowing in and out respectively; rho shell ，C p,shell ，T shell,i The density, the constant pressure specific heat and the temperature of the ith control volume of the shell are respectively; q r,i And Q w,i The heat exchange amount and the medium side of the refrigerant side and the shell respectivelyThe amount of heat exchange with the shell; k is a radical of formula r,i And k w,i Heat exchange coefficients of the refrigerant side and the medium side, respectively; a. The r,i And A w,i Heat transfer areas for the refrigerant side and the media side, respectively; t is r,i And T w,i Temperatures of the ith control volumes, respectively, the refrigerant side and the media side; t represents time.
In equation (7), the governing equation of the controlled volume on the refrigerant side is considered:
compared with the traditional modeling mode, the calculation precision can be improved, the modeling mode of the phase change heat exchanger control volume based on the Modelica modeling language only needs to dynamically model one control volume, the modeling complexity is reduced, and the calculation speed is guaranteed.
S14, dividing the solution heat exchanger into a plurality of control volumes with equal length along the fluid flow direction, wherein each control volume follows mass conservation, energy conservation and momentum conservation, and establishing a solution heat exchanger model as follows:
in the formula (8), the reaction mixture is,
and
mass flow rates of solution flowing into and out of the ith control volume, respectively;
and
mass fractions of the influent and effluent solutions, respectively; v sol,i Is the control volume of the solution heat exchanger;
and
specific enthalpy of the influent and effluent solutions, respectively; rho sol,i ，p sol,i ，h sol,i Density, pressure and specific enthalpy in the ith control volume in the solution heat exchanger, respectively;
and
the pressure of the solution flowing in and out respectively; q sol,i The heat exchange quantity of the ith control volume in the solution heat exchange; k is a radical of sol,i And A sol,i The heat transfer coefficient and the heat transfer area of the control volume are respectively; delta T sol,i The temperature difference of the fluid on the two sides of the ith control volume of the solution heat exchanger; t represents time.
In equation (8), the governing equation of the controlled volume on the refrigerant side is:
compared with the traditional modeling mode, the calculation accuracy can be improved, and the modeling mode of the control volume of the solution heat exchanger based on the Modelica modeling language only needs to dynamically model one control volume, so that the modeling complexity is reduced, and the calculation speed is ensured.
S15, establishing a solution pump and valve model according to a flow calculation formula of the solution pump and the valve as follows:
in the formula (9), the reaction mixture is,
and
mass flow rates of the solution pump and the valve respectively; n is p ，V p The rotation speed and the volume displacement of the solution pump are respectively; rho pump Is the solution density entering the solution pump; u and S are respectively the opening degree and the cross-sectional area of the valve; p is a radical of h And p l Respectively high pressure and low pressure at two sides of the valve; ρ is a unit of a gradient valve Is the density of the fluid entering the valve.
The valve includes a solution regulating valve and a refrigerator throttle valve.
And S2, respectively establishing connectors aiming at different physical domains in the lithium bromide absorption refrigeration system, wherein the connectors comprise a solution connector, a water connector and a thermal connector and are used for calculating variables of the different physical domains and transmitting the variables among component models.
Specifically, variables that may be present in each connector include flow-through variables, potential variables, and flow variables. Wherein, the flow variable in the solution connector is the solution mass flow, the potential variable is the pressure, and the flow variable is the temperature and the mass fraction of the solution; the flow variable in the water connector is the mass flow of the refrigerant, the potential variable is the pressure, and the flow variable is the specific enthalpy of the refrigerant; the flow variable in the thermal connector is the heat flow and the potential variable is the temperature.
And S3, according to the structure of the lithium bromide absorption refrigeration system, adding the corresponding connectors to each component model to enable the component models to have the function of exchanging variables, and correspondingly connecting the connectors of the same type in each component model to form a dynamic simulation model.
Specifically, the connectors are classified into two types, inflow and outflow. The boundary conditions of the flow variables into and out of the connectors differ when applied in the respective assemblies and are also differentiated on the graphical interface, the flow direction being taken into account when using these connectors.
Specifically, connectors are added in the component model, the components are connected through graphical modeling, and the connection relationship among the connectors is shown in fig. 2.
As shown in fig. 2, one solution inflow connector, one solution outflow connector, two water inflow connectors and one water outflow connector are added to the generator model; adding a solution inflow connector, a solution outflow connector, a water inflow connector and two water outflow connectors into the absorber model; two water inflow connectors and two water outflow connectors are respectively added in the evaporator model and the condenser model; a solution inflow connector and a solution outflow connector are added into the solution pump and the solution regulating valve model; a water inflow connector and a water outflow connector are arranged in the throttle valve model; the solution heat exchanger was charged with two solution inflow connectors and two solution outflow connectors. Wherein, a water outlet connector of the generator means that the outlet of the generated steam is connected with the water inlet connector of the condenser, after the refrigerant steam passes through the throttle valve, the water flowing into the evaporator flows into the connector, and finally the water flowing into the absorber flows into the connector, thus completing the refrigerant circuit. The concentrated solution flows from a solution outflow connector of the generator, passes through the solution heat exchanger and the solution regulating valve and then enters the solution inflow connector of the absorber, and the dilute solution in the absorber flows from the solution outflow connector, passes through the solution pump and the solution heat exchanger and then enters the generator, so that a solution loop is completed.
As shown in FIG. 2, the input parameters of the constructed dynamic simulation model include the generator pipe side heat source water flow
And generator tube side heat source water inlet temperature
Chilled water inlet flow on heat exchange medium side of evaporator
And chilled water inlet temperature
Absorber tube side cooling water flow
And absorber tube side cooling water inlet temperature
Valve opening u and rotation speed n of solution pump p (ii) a The output parameters of the constructed dynamic simulation model comprise the heat exchange quantity Q in the condenser con Heat exchange quantity Q in evaporator evp Heat exchange quantity Q in solution heat exchanger hex Outlet temperature of chilled water on heat exchange medium side of evaporator
Heat exchange quantity between generator shell side and tube wall
Heat exchange quantity between absorber shell side and tube wall
S4, determining input parameters under the design working condition, and setting system structure parameters including the volume displacement of the solution pump of 3.8E-07m when the input parameters are consistent 3 The cross section area of the solution regulating valve is 9.6E-06m 2 The cross-sectional area of the throttle valve is 2.5E-05m 2 The volume of the generator and the absorber is 0.024m 3 The volume of the evaporator is 0.002m 3 The volume of the condenser is 8.4E-04m 3 The volume of the solution heat exchanger is 5E-04m 3 . Carrying out simulation calculation to obtain an output result of the dynamic simulation model, comparing the result of the dynamic simulation model under the design condition with an output value of an actual system, when the temperature step of the heat source rises by 10 ℃, comparing the change conditions of the system energy efficiency and the chilled water outlet temperature with the output value of the actual system, wherein the change rules are similar as shown in (a) and (b) in fig. 3, and the parameter values when reaching the steady state can be well matched.
The accuracy of the dynamic simulation model of the embodiment is higher.
And if the accuracy does not reach the preset standard, correcting the dynamic simulation model by adjusting the system structure parameters.
S5, carrying out dynamic simulation research: and performing a step change experiment on the input parameters of the dynamic simulation model to obtain the dynamic response of the output parameters of the lithium bromide absorption refrigeration system. In the dynamic simulation research process, modelica language can be used for recording the characteristics of intermediate variables in the dynamic process, and the reasons for forming the dynamic characteristics are analyzed through dynamic response to research the dynamic characteristics of the lithium bromide absorption refrigeration system and provide reference for the design of the system and the controller. The method specifically comprises the following steps:
s51: setting the initial temperature of heat source water to 92 ℃ and the initial mass flow to 0.9m 3 And h, before 15000s, carrying out step change on the mass flow every 5000s, and after 20000s, carrying out step change on the heat source temperature to analyze the dynamic response of the steam flow generated by the generator, the generator power, the outlet temperature of the chilled water and the system energy efficiency.
The step change of the heat source water and the dynamic response of the generator parameters are shown in (a), (b) of fig. 4. An increase in heat source water flow or temperature will result in an increase in generator power, causing it to produce more refrigerant vapor. It is noted that the steam flow and generator temperature may be characterized as overshooting and initially reversing when the hot water supply conditions change.
This is due to the fact that the temperature in the generator gradually increases as the generator power increases, reducing the heat transfer temperature difference, and the generator power decreases again. Therefore, the power of the generator is over-regulated, and the generated steam has the same trend. Similarly, when the temperature or flow of hot water entering the generator is reduced, the power of the generator is sharply reduced, the temperature of the generator is reduced, the heat transfer temperature difference is increased, and finally the initial reverse phenomenon occurs to the power of the generator and the steam flow.
As shown in fig. 5, the system energy efficiency and the generator power of the dynamic simulation model of the lithium bromide absorption refrigeration system have opposite trends, and the variation trend of the outlet temperature of the chilled water is similar to the variation trend of the generator power. When the heating condition of the generator becomes good, the outlet temperature of the chilled water rises, and the response time of the rise also varies, and is about 3750 seconds at 5000 seconds and 2750 seconds at 15000 seconds. This is due to the fact that the larger the steam flow, the longer the equilibrium time required to accumulate mass and energy.
S52: the flow rate of the heat source water was set to 0.9m 3 H, the temperature of the heat source water is set to be 92 ℃, and the initial flow rate of the cooling water is 2.5m 3 Step change every 5000 s; and after 20000s, keeping the flow of cooling water unchanged, setting the temperature of initial cooling water to be 30 ℃, carrying out step change on the initial cooling water every 5000s, and analyzing the dynamic response of the generated steam flow, the absorber power, the chilled water outlet temperature and the system energy efficiency.
The change of state of the cooling water and the response of the generated steam flow and the absorber power are shown in (a), (b) of fig. 6. It can be seen that as the cooling water flow increases or the temperature decreases, the vapor flow increases because both improve the cooling conditions of the lithium bromide absorption refrigeration system. The temperature in the absorber can be reduced under better cooling conditions, and the absorption capacity of the solution is improved, so that the steam flow is increased; as the absorber temperature decreases, the heat transfer differential decreases and the absorber temperature reaches a new steady state value. Therefore, the generated steam flow rate is overshot. Similarly, due to the heat transfer process in the absorber, the steam flow will exhibit an initial reversal behavior as the cooling conditions deteriorate. The absorber power changes follow this law as well.
The dynamic response of chilled water outlet temperature and system energy efficiency is shown in fig. 7. It can be noted that improving the cooling conditions can increase the system energy efficiency while reducing the chilled water outlet temperature. Compared with the dynamic response of changing the heat source water, the energy efficiency of the system changes more smoothly because the cooling water indirectly affects the generator power and the cooling power. The heat transfer process also causes initial reversal and overshoot of the system energy efficiency and chilled water outlet temperature
The method and the system utilize the characteristics of Modelica language object-oriented and graphical modeling to model the lithium bromide absorption refrigeration system comprising multiple physical domains and model the connector, and combine the components of the system into a dynamic simulation model through the connector. Compared with the traditional modeling mode, the method does not need to carry out mathematical processing on the dynamic equation, and can modify and debug models such as a generator and an absorber more conveniently.
According to the modeling method of the phase change heat exchanger and the solution heat exchanger, the calculation precision is improved, the calculation speed is guaranteed, only one control volume needs to be dynamically modeled, and the modeling complexity is reduced.
The method and the device can record the characteristics of each intermediate variable in the dynamic process by using the Modelica language, and improve the convenience of analyzing the dynamic characteristics of the lithium bromide refrigerator.
Those of ordinary skill in the art will understand that: although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that changes may be made in the embodiments and/or equivalents thereof without departing from the spirit and scope of the invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (8)
1. A dynamic simulation method of a lithium bromide absorption refrigeration system based on Modelica language is characterized by comprising the following steps:
the method comprises the steps that a lithium bromide absorption refrigeration system is split into different components according to functions, modeling is carried out on each component through a Modelica modeling language based on heat exchange, mass conservation, energy conservation and momentum conservation to form a component model, wherein the component model comprises a generator model, an absorber model, a phase-change heat exchanger model, a solution pump and a valve model, and the phase-change heat exchanger model is suitable for describing an evaporator and a condenser;
respectively establishing connectors aiming at different physical domains in the lithium bromide absorption refrigeration system, and calculating variables of the different physical domains and transmitting the variables among the component models;
adding corresponding connectors on each component model, and correspondingly connecting the connectors of the same type in each component model according to the structure of the lithium bromide absorption refrigeration system to form a dynamic simulation model;
setting structural parameters of the system according to input parameters under the design working condition, carrying out simulation calculation under the design working condition, comparing a simulation result with an output result of an actual system, verifying a dynamic simulation model, and correcting the dynamic simulation model by adjusting the structural parameters;
carrying out dynamic simulation research: and performing a step change experiment on the input parameters of the dynamic simulation model to obtain the dynamic response of the output parameters of the lithium bromide absorption refrigeration system.
2. The Modelica language-based dynamic simulation method for the lithium bromide absorption refrigeration system according to claim 1, wherein the modeling of each component through the Modelica modeling language comprises:
establishing a generator model:
establishing a generator shell side dynamic model according to an energy conservation equation of the generator shell side and a heat exchange equation of the shell side and a pipe wall, wherein a pushing work term is considered in the energy conservation equation of the generator shell side:
wherein M is gen ，h gen ，p gen Respectively the accumulated mass, solution specific enthalpy and pressure in the generator;
respectively, dilute solution mass flow, concentrated solution mass flow and generated steam mass flow; v gen Is the generator volume; h is weak ，h strong ，h v,gen Respectively dilute solution specific enthalpy, concentrated solution specific enthalpy and generated steam specific enthalpy;
is a generatorThe heat exchange quantity between the shell side and the tube wall; t represents time;
establishing a generator tube side dynamic model according to an energy conservation equation of a generator tube side fluid and a heat exchange equation of the tube side fluid and a tube wall;
establishing a generator pipe wall dynamic model according to an energy conservation equation of the generator pipe wall;
and combining the generator shell side dynamic model, the generator tube side dynamic model and the generator tube wall dynamic model into the generator model.
3. The Modelica language-based dynamic simulation method for the lithium bromide absorption refrigeration system according to claim 1, wherein the modeling of each component through the Modelica modeling language comprises:
establishing an absorber model:
establishing an absorber shell side dynamic model according to an energy conservation equation of the absorber shell side and a heat exchange equation of a shell side fluid and a pipe wall, wherein a pushing work term is considered in the absorber shell side energy conservation equation:
wherein M is abs ，h abs ，p abs Respectively the accumulated mass, the solution specific enthalpy and the pressure in the absorber;
respectively the mass flow of the dilute solution, the mass flow of the concentrated solution and the mass flow of the absorbed steam; v abs Is the absorber volume; h is v,abs To absorb specific enthalpy of steam;
the heat exchange quantity between the shell side of the absorber and the tube wall is obtained; t represents time;
establishing an absorber tube side dynamic model according to an energy conservation equation of absorber tube side fluid and a heat exchange equation of the tube side fluid and a tube wall;
establishing an absorber tube wall dynamic model according to an energy conservation equation of the absorber tube wall;
and combining the absorber shell side dynamic model, the absorber tube side dynamic model and the absorber tube wall dynamic model into the absorber model.
4. The Modelica language-based dynamic simulation method for the lithium bromide absorption refrigeration system according to claim 1, wherein the modeling of each component through the Modelica modeling language comprises:
establishing a phase change heat exchanger model:
dividing the phase change heat exchanger into a plurality of control volumes with equal length along the fluid flow direction, wherein each control volume follows mass conservation, energy conservation and momentum conservation, and the control equation of the control volume at the refrigerant side of the phase change heat exchanger is as follows:
wherein, V r,i A control volume on the refrigerant side; rho r,i ，p r,i ，h r,i Density, pressure and specific enthalpy in the ith control volume on the refrigerant side, respectively;
and
mass flow rates into and out of the ith refrigerant side control volume, respectively;
and
specific enthalpy of the refrigerant flowing in and out, respectively;
and
the refrigerant pressure of the inflow and outflow, respectively; q r,i The heat exchange quantity of the refrigerant side and the shell; t represents time.
5. The Modelica language-based dynamic simulation method for the lithium bromide absorption refrigeration system according to claim 1, wherein the modeling of each component through the Modelica modeling language comprises:
establishing a solution heat exchanger model:
dividing the solution heat exchanger into a plurality of control volumes of equal length along the direction of fluid flow, each control volume following conservation of mass, energy and momentum, the governing equation for the control volume on the refrigerant side of the solution heat exchanger being:
wherein,
and
mass flow rates of solution flowing into and out of the ith control volume, respectively;
and
mass fractions of the influent and effluent solutions, respectively; v sol,i Is the control volume of the solution heat exchanger;
and
specific enthalpy of the influent and effluent solutions, respectively; rho sol,i ，p sol,i ，h sol,i Density, pressure and specific enthalpy in the ith control volume in the solution heat exchanger respectively;
and
the pressure of the solution flowing in and out respectively; q sol,i The heat exchange quantity of the ith control volume in the solution heat exchange; t represents time.
6. The Modelica language based dynamic simulation method for the lithium bromide absorption refrigeration system according to claim 1, wherein the modeling of each component through a Modelica modeling language comprises:
and establishing a solution pump and valve model through a flow calculation formula of the solution pump and the valves, wherein the valves comprise a solution regulating valve and a refrigerating machine throttling valve.
7. The Modelica language based lithium bromide absorption refrigeration system dynamic simulation method according to claim 1, wherein the connectors include a solution connector, a water connector and a thermal connector, the flow variable in the solution connector is a solution mass flow rate, the potential variable is a pressure, the flow variable is a temperature and a mass fraction of the solution, the flow variable in the water connector is a mass flow rate of the refrigerant, the potential variable is a pressure, the flow variable is a specific enthalpy of the refrigerant, the flow variable in the thermal connector is a thermal flow rate, and the potential variable is a temperature.
8. The Modelica language based dynamic simulation method for a lithium bromide absorption refrigeration system according to claim 1, wherein the input parameters include generator tube side heat source water flow and generator tube side heat source water inlet temperature, chilled water inlet flow and chilled water inlet temperature on the evaporator heat exchange medium side, absorber tube side cooling water flow and absorber tube side cooling water inlet temperature, valve opening and solution pump rotational speed; the output parameters comprise the heat exchange quantity in the condenser, the heat exchange quantity in the evaporator, the heat exchange quantity in the solution heat exchanger, the chilled water outlet temperature at the heat exchange medium side of the evaporator, the heat exchange quantity between the shell side of the generator and the tube wall, and the heat exchange quantity between the shell side of the absorber and the tube wall.
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